1. Field of the Invention
This invention relates to an exhaust gas purifying catalyst.
2. Discussion of the Background
A ferrite heat resistance stainless steel plate containing aluminum has been used as a catalyst support for an automobile exhaust gas purifying catalyst. One of the ferrite heat resistance stainless steel plates is disclosed in Japanese Examined Patent Publication (KOKOKU) No. 23138/1983, and contains the following: 3 to 40% by weight of chromium; 1 to 10% by weight of aluminum; 1 to 5% by weight of cobalt; 0 to 72% by weight of nickel; 0 to 0.5% by weight of carbon; and the balance of iron. The surface of this ferrite heat resistance stainless steel plate is oxidized to form an alumina film.
Further, Japanese Unexamined Patent Publication (KOKAI) No. 68143/1982 discloses an improvement on an activated alumina catalyst carrier layer forming method on the alumina film of a catalyst support. In this method, the surface of Fe-Cr-Al-Ti-Zr alloy catalyst support is covered with a film which contains TiZr alloy solidifying in alpha-alumina or a film which contains alpha-alumina as a major component and oxides of titanium (Ti) and zirconium (Zr) as minor components.
Furthermore, Japanese Unexamined Patent Publication (KOKAI) No. 71898/1982 discloses a method, in which the heat resistance ferrite stainless steel plate is oxidized with a small volume of oxygen to form alumina whiskers on the surface of the catalyst support to strengthen the adhesion between the activated alumina catalyst carrier layer and the catalyst support.
The conventional techniques form an alumina film on the ferrite heat resistance stainless steel plates containing aluminum by heat or chemical treatment. The ferrite heat resistance stainless steel plates covered with the alumina film are used for catalyst supports, since it has been believed that the alumina film suppresses the propagation of the oxidation into the catalyst supports and increases the adhesion strength between the catalyst supports and the activated alumina catalyst carrier layer to be coated thereon.
However, it is hard to cover the ferrite heat resistance stainless steel plates with the alumina film evenly by the conventional techniques since the alumina film is produced out of the aluminum contained in the ferrite heat resistance stainless steel plates. As the aluminum in the ferrite heat resistance stainless steel plates is consumed, for instance, to 2% by weight or less based on the total weight of the ferrite heat resistance stainless steel plates, chromium and iron become susceptible to oxidation. In addition, as thus formed alumina film transforms from one state to the other, the alumina film is come off the ferrite heat resistance stainless steel plates and the come-off portions are likely to be oxidized.